Why do juniors swap so readily? I think the tension gets to them. They
have learned that an unresolved tension (Nd4-Nc6) might be overlooked
later on (Qf3?? Nxd4 or Nc3 d6; Bc4 b6??; Nxc6+ or Bc5 Be3 Nge7?? Nxc6).
But swapping can also be a mistake. I think 4...Nxd4? just gives White
better chances, but I also think 3...exd4 is more or less forced. So,
how do you judge when to keep, and when to release, the tension? When
do you seek to swap, and when to hold steady?

"The ability to create and to control the tension of battle is perhaps
the principal attainment of the great player." -- Savielly Tartakower

The tension formation (Euwe-Kramer)

1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 6. Re1 b5 7. Bb3 O-O
8. c3 d6 9. h3 Na5 ( 9... Bb7 10. d4 ) 10. Bc2 c5 11. d4 {One of the
classic formations, under debate and contest for over a century. The
correct placement of the pieces will be different, depending on the
central pawn formation. But the central pawn structure is not yet
fixed! It might turn out to be symmetrical, or open, or closed, or...
So where do you put your pieces? And what do you do in the meantime? }
*

Many top-level opening choices have some sort of unresolved tension like
this. In fact, following Carlsen, the aim of the opening may not be a
fruitless search for an advantage as White or equality as Black, but
a position which gives you chances to outplay your opponent.

"The whole concept has changed! Now the main concept is “to get a
playable position and maintain the tension”." - Sergey Karjakin

1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 6. Re1 b5 7. Bb3 d6
8. c3 O-O 9. h3 { We've been here before...} 9... Bb7 { The so-called
Zaitsev Variation, which has been all the rage since it was taken up by
Anatoly Karpov.} 10. d4 Re8 11. Nbd2 Bf8 12. a4 { The critical move,
but it is very tactical and requires lots of preparation.} 12... exd4
13. cxd4 Nb4 { leads to very complex positions which were thoroughly
explored in the Kasparov - Karpov matches.[VM] Black exposes the centre
and relies on piece activity for chances -- rather a hypermodern
approach.[DR] "Chess is awful. If you don't have the centre you have
difficulties... And if you do have the centre, you really do have
problems!" -- Tarrasch.} *

Some closing thoughts

"Never make a good move too soon" - James Mason

"The threat is stronger than its execution." Nimzowitsch

"Sometimes the hardest thing to do in a pressure situation is to allow
the tension to persist. The temptation is to make a decision, any
decision, even if it is an inferior choice." -- Garry Kasparov

"A swap is rarely neutral - it will favour one side or the other.
So make sure it's you!" -- Me